A SWARM OF PEOPLE are going to walk 14 miles today because 25 years ago this newspaper was wrong. The mistake occurred on Jan. 3, 1954, when The Post Endorsed making a parkway out of the old Chesapeake and Ohio Canal.

For newcomers to the area or the subject, here is a recap of what happened then. The editorial provoked Supreme Court Justice William O. Douglas' eloquent invitation to hike the 185-mile towpath with him and see why this "long stretch of quiet and peace" should remain untouched. Two editorial writers went. By the time they stumbled back in town, the "Douglas hike" had persuaded them and others that the neglected right-of-way indeed should be kept as a refuge of nature, history and calm. Many hikes and arguments later, Congress concurred. In 1971 the canal became a national historical park. And the C&O Canal Association's annual one-day hikes have grown even more popular as footloose celebrations of the park and of spring.

Now it's that season again. To mark the 25th anniversary, about three dozen sturdy folk have spent the last two weeks hiking from Cumberland. Today they will be joined by hundreds more for the final 14 miles from Great Falls, Md., (where they stride out at 9 a.m.) to the tidal lock (or what's left of it) at Thompson's Boathouse in town.

It's a fine chance to stretch your muscles and to consider, as you stroll, all that has happened in that area since 1954. In mid-morning, for instance, you'll walk under the Beltway. Think of what that has meant for this sprawling metropolis. Later, you'll be surrounded by the in-town boom, from Rosslyn's high-rises across the river to Georgetown's construction and crowds. And when you finally stop beside the Potomac, you can reflect on the additions to that monumental scene since 1954-including the Kennedy Center, the Roosevelt Bridge and (unavoidably) the Watergate.

Besides the evidence of growth, the route includes signs of three victories for conservation and sense. At Little Falls, the Corps of Engineers is building a pump to draw some drinking water from the estuary. That's one fruit of the fight against upriver dams. Another recent battleground is the now-placid spot where the Three Sisters Bridge was going to be. Third, thanks to great regional efforts, the river itself is much cleaner than before.

Finally, the fact that anyone can walk-or jog or bike or sit-along a green, protected river bank, right next to town, is a tribute to public foresight and environmental care. Of course, there's always more to do for the canal, the river and the rest of the area's wealth of trails and parks. But for one April day, it's good simply to hail and enjoy a path that reaches so far into the hills and ensures that so much of the upstream Potomac shore will stay open, uncluttered and unpaved.